1. Field of the Invention
The present invention relates to an organic electroluminescent device (light-emitting device or EL device) capable of converting electrical energy into light and emitting light.
2. Description of the Related Art
Nowadays, research into various display devices is active, among which organic electroluminescent (EL) devices are attracting attention as promising display devices that achieve high brilliance of emitted light at a low voltage.
Generally, organic EL devices have a configuration having opposing electrodes (i.e., anode and cathode), and, between the opposing electrodes, a light-emitting layer or a plurality of organic layers containing the light-emitting layer. In the organic EL devices, electrons injected from the cathode and electron holes injected from the anode are recombined in the light-emitting layer, and the resultant excitons emit light, or energy from the excitons migrates to other molecules to generate excitons, which emit light.
However, further improvements in luminous efficiency and improved brilliance have been strongly demanded with a view to energy-saving or improved durability.
It is conceivable that organic EL devices can be used, for example, as a white light source since organic EL devices are individually-luminescing surface light sources. An ideal light source for white light, as defined by the Commission Internationale de l'Eclairage (CIE), has coordinates of (0.33, 0.33) on the CIE Chromaticity Diagram. It is possible to achieve the emission of white light from the luminescence of blue, green, and red light-emitting materials, or from the luminescence of materials which emit rays of two complementary colors.
For a device emitting white light, the emission of white light at low voltage with high brilliance and chromaticity is desired. The use of phosphorescent materials having a luminous efficiency that is higher than fluorescent materials is desired for reduced voltage and higher brilliance (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2001-319780, 2004-281087 and 2004-522276). In particular, the development of a phosphorescent material of blue luminescence and the development of a device for making the blue-luminescence phosphorescent material emit light effectively has been desired for improvement in luminous efficiency of light-emitting devices. The reason for this is that, if the luminescent intensity of the blue color is low, it becomes necessary to reduce and adjust the phosphorescent luminescent intensities of the green color and the red color, which are known to be emitted with high efficiency, in order to achieve a desired chromaticity, resulting in reductions in the luminous efficiency of these light-emitting devices.
Further, when the transmission of energy (blue color→green color→red color) between the light-emitting materials occurs in a white light-emitting device, chromaticity deteriorates, and therefore improvement has been desired. A white light-emitting device containing a phosphorescent light-emitting material in each of the blue, green and red light-emitting layers has been disclosed in JP-A No. 2004-522276. However, the luminous efficiency and chromaticity are still inadequate, and further improvement has been desired.
Therefore, there is a need for an organic electroluminescent device having excellent luminous efficiency and chromaticity.